1. Field of the Invention
The present invention relates to transmission cables for transmitting a plurality of signals, and in particular, relates to transmission cables for performing multiple differential transmission in which a plurality of data bits is achieved through differential transmission, which is one method of high-speed signal transmission, with a differential transmission line that is provided with a small number of signal lines.
2. Description of the Related Art
Single-end signals that oscillate with a logical amplitude at the power source voltage have conventionally been used for the transmission of high-speed signals, but the use of low voltage differential signaling (LVDS) has been on the rise in light of the increase in the number of drive frequencies and larger bus widths in conjunction with the recent demand for high-speed data transmission, as well as because of its inhibition of unwanted radiation noise and resistance against exogenous noise. With LVDS, generally the differential driver IC is designed such that only a reverse-phase differential mode current flows between the two transmission lines through which the differential signal flows.
FIG. 25 is a circuit diagram of the differential transmission circuit according to a first conventional technology, and FIG. 26 is a perspective view showing the schematic configuration of the differential transmission circuit of FIG. 25. The differential transmission circuit of FIG. 25 is an example of a conventional LVDS interface configuration. As shown in FIG. 26, a differential driver IC911 is provided on a printed circuit board 914a and a differential receiver IC913 is provided on another printed circuit board 914b that is away from the printed circuit board 914a, and the differential driver IC911 and the differential receiver IC913 are connected by a differential transmission cable 912 serving as a differential transmission line that is provided with a positive signal line 912a and a negative signal line 912b (the other circuit elements on the printed circuit boards 914a and 914b have been omitted from the drawings). Thus, a bit information signal that is input to the differential driver IC911 is transferred to the differential receiver IC913 via the differential transmission cable 912 and then output.
In FIG. 25, the positive output terminal of the differential driver IC911 (in FIG. 25, represented by point p1) is connected to the positive input terminal of the differential receiver IC913 via the signal line 912a, and similarly, the negative output terminal of the differential driver IC911 (in FIG. 25, represented by point p2) is connected to the negative input terminal of the differential receiver IC913 via the signal line 912b. To terminate the differential transmission cable 912, the point p3 where it approaches the differential receiver IC913 on the signal line 912a and the point p4 where it approaches the differential receiver IC913 on the signal line 912b are connected by a 100-Ω terminal resistor R. The differential transmission cable 912 has a 50-Ω odd mode impedance. The positive signal line 912a and the negative signal line 912b of the differential transmission cable 912 have equal electrical properties and form equivalent transmission routes, and in LVDS these two signal lines 912a and 912b together effect the transmission of a single bit information signal. Based on the bit information signal that is input from its input terminal, the differential driver IC911 creates a differential signal that causes a potential difference between the positive and negative sides of the differential transmission cable 912. More specifically, the differential driver IC911 drives an approximately 3.5 mA current in order to generate an approximately 350 mV voltage between points p3 and p4 at either end of the 100-Ω terminal resistor R. The differential receiver IC913 detects the approximately 350 mV differential signal that is produced between points p3 and p4 at either end of the terminal resistor R and converts this to a CMOS level (a voltage level about 20 to 40% of the power source voltage; same hereinafter) and outputs the converted bit information signal from the output terminal.
In LVDS, the signal currents Is that flow through the positive signal line 912a and the negative signal line 912b of the differential transmission cable 912 are the same size and are in opposite directions, and thus unwanted radiation noise and crosstalk noise are kept from occurring due to the fact that the magnetic fields that are produced by these moving currents cancel each other out, and also because the signal level is small. With regard to exogenous noise also, as long as the positive and negative sides of the differential transmission cable 912 are affected relatively similarly, then there is no effect on the logic value of the signal, and thus LVDS has excellent noise resistance as well. However, the flow of a tiny in-phase common mode current to the differential transmission cable 912 occurs in LVDS, as well as due to differential impedance mismatching in the differential transmission line, such as the printed circuit board or the transmission cable, or the end terminal circuit, and skewing between the signal lines 912a and 912b of the differential transmission cable 912. In the differential transmission cable 912 in FIG. 25, the differential mode current component is matched by the terminal resistor R and terminated, but there is no route for the common mode current component to flow over the circuit, and it returns via stray capacitance on the printed circuit boards 914a and 914b, for example. Thus, the common mode current component that is generated in the differential transmission cable 912 was the primary source of unwanted radiation noise that radiates from LVDS transmission systems. In order to solve this issue, the two signal lines 912a and 912b are laid out parallel to and near one another as shown in FIG. 26, preventing differential impedance mismatch (for example, see Japanese Laid-Open Patent Publication No. 2001-267701). With this method, the common mode current that flows to the differential transmission cable 912, which is made from the two signal lines 912a and 912b, is inhibited, allowing transmission noise and unwanted radiation noise to be inhibited.
However, compared to ordinary single-end transmission, the differential transmission circuit of FIGS. 25 and 26 has the numerous above-described merits with regard to high-speed transmission, but requires the two signal lines 912a and 912b in order to transmit a single data bit, and this leads to problems such as requiring a large number of signal lines in order to achieve multiple bit transmission, a thicker differential transmission cable 912, and an increase in the wiring region on the printed circuit boards 914a and 914b. One method that has been conceived for solving this problem is the use of three signal lines, with one of the signal lines serving as a complementary data line, in order to achieve transmission of two data bits with three signal lines, which required four signal lines in conventional differential transmission (for example, see Japanese Patent No. 3507687).
FIG. 27 is a perspective view that schematically shows the configuration of the differential transmission circuit according to a second conventional technology, and FIG. 28 is a horizontal cross section of the differential transmission cable 912A in FIG. 27. A differential driver IC911A and a differential receiver IC913A are connected by a differential transmission cable 912A, which is made of three signal lines 912a, 912b, and 912c. A first bit information signal that is input to the differential driver IC911A is transmitted to the differential receiver IC913A over the signal lines 912a and 912b, and similarly, a second bit information signal that is input to the differential driver IC911A is transmitted to the differential receiver IC913A over the signal lines 912b and 912c. A terminal resistor for terminating the signal lines 912a and 912b, and a terminal resistor for terminating the signal lines 912b and 912c, are provided in the differential receiver IC913A. When the three signal lines 912a, 912b, and 912c of the differential transmission cable 912A are disposed as shown in FIG. 28, the distances between the two adjacent signal lines 912a and 912b, and 912b and 912c, and the signal lines 912a and 912c on the sides, are different (L1≠L3, L2≠L3), and this creates a novel problem in that the electromagnetic fields cannot cancel each other out because a constant differential impedance cannot be maintained, and unwanted radiation noise cannot be reduced.